Phylum: Charophyta   Family: Closteriaceae

Closterium idiosporum  West & G.S. West 1900

The variety punctatum can only be reliably separated when found with the zygospores; rectangular in var. punctatum, elliptical in the nominate. Common in acidic moorland pools and loch-margins, also with Sphagnum.

References: 
West, W. & West, G.S. (1900) Notes on freshwater algae. II.
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.